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December 2013

12/11/2013

In mid-July of 2013, the Deep Time exhibits team went to North Dakota to collect fossils from the Hell Creek Formation. Our goal was to find 66-million-year-old fossils from the Late Cretaceous for our new exhibitions, and to learn more about paleontology and deep time. This is the fourth post in a series about our experiences out in the field.

Shortly into our expedition I found myself asking “what the Hell Creekis the K/Pg boundary, exactly?” I knew it had something to do with the extinction of the dinosaurs, the Chicxulub asteroid impact, and the transition from the Cretaceous to the Paleogene periods about 66 million years ago, but I still had so many questions. Was the boundary a physical thing, or a human construct? Did it have to do with rock formations, with a point in absolute time, or a specific event? Luckily I was surrounded by curators who specialize in this area of paleontology, so I was soon swimming in answers.

The Cretaceous-Paleogene boundary, now called the K/Pg but formerly known as the Cretaceous-Tertiary or K/T boundary, is essentially a place-marker. Scientists look at physical evidence and choose a certain event —a mass extinction, the sudden presence of certain elements, or the appearance of a specific species —as worthy of being a dividing moment in time. In other words, a boundary is based on something tangible, but we assign its value.

Who chooses a boundary like the K/Pg, and what does it mean?

The International Geologic Congress (IGC) is the scientific body that selects the “stratotype,” or the exemplary occurrence of the physical markers of a boundary. In the most technical sense, the IGC defined the K/Pg boundary in a section of rock in El Kef, Tunisia. There’s even a marker called the “golden spike!” In a broader, more dramatic perspective, the K/Pg boundary marks the moment that an asteroid hit the Earth in the Yucatán Peninsula, causing massive ecological changes and leading to the extinction of dinosaurs and many other life forms. It’s the moment that takes us out of the Cretaceous Period and into the Paleogene Period.

What’s so special about the El Kef section?

It contains well-preserved microfossils as well as geochemical and mineralogical markers of the asteroid impact, enabling scientists to compare and correlate it with other layers of rock around the world. Think of a stratotype like a “type specimen” of rock layer: just as biologists choose an individual specimen from every plant and animal species as the ultimate “ground truth” to represent that whole species, geologists select representative rock sections to which all other sections are compared.

For example, if you discover a dinosaur skull that you think belongs to a Tyrannosaurus rex, it’s on you to make a case that it’s morphologically similar enough to qualify. And if you think you’ve found the K/Pg boundary somewhere else in the world (say, in a butte in North Dakota), you have to prove that your location has enough in common with the El Kef site to qualify as a correlate of the boundary.

The key markers of the K/Pg boundary are:

-increased iridium presence. Iridium is rare on Earth’s crust, but common in asteroids and comets. A global spike in iridium in a thin layer of sediment was our first clue that an asteroid had struck the Earth about 66.5 million years ago.

-tiny balls of clay called spherules. These are aged versions of microtektites — tiny, half-millimeter-sized glass beads that formed when molten droplets of rock, launched from the impact crater, cooled as they flew through the air. Scientists find spherules worldwide, but in sites closer to the impact crater you can still find microtektites in their original glass form.

-shocked quartz. Under intense pressure, like from a nuclear bomb or an asteroid impact, the crystalline pattern of quartz gets disrupted, making planes that show up as lines under a microscope. Like spherules, shocked quartz is found worldwide in the K/Pg boundary layer.

-evidence of tsunamis. The 6-mile wide asteroid caused massive tidal waves when it hit near the Yucatán Peninsula. In some areas of the world, the rock strata record ocean sand in what should have been a terrestrial environment, or chaotically mixed sediment with terrestrial plant debris in what should have been a sea.

-species extinction and fossil turnover. Right below the K/Pg boundary layer, you’ll find certain kinds of fossil plants, animals, and pollen, but right above the boundary they all disappear from the fossil record.

Sometimes the boundary layer is hard to find and you don’t know you have it until you’ve taken samples back to the lab to analyze for iridium and pollen that records the big changes in the land flora. Other times, you have a pretty good idea that you’ve hit the jackpot, since you can see fossil turnover and spherules with your bare eyes (though for the spherules, you have to look really closely). At one butte in particular, the K/Pg boundary was easy to spot, and we could see the spherules up close. In fact, the spherules give the clay distinctive texture – like puffed rice in a chocolate bar, except less delicious.

Caption: Care for a piece of K/Pg boundary? Mmm, taste that terroir… Photo by Siobhan Starrs

Finding the K/Pg boundary, standing on it, even tasting it, was a humbling experience because it put our human existence into perspective. We are a very recent addition to the tree of life, and the world has suffered and recovered from catastrophes we could hardly imagine. But perhaps the thing that amazes me the most is that our species has figured out a way to read the history of our planet in the rocks beneath our feet.